A computerized method determines similarities between a first image and a second image. The first image is converted into a first grayscale image, and the second image is converted into a second grayscale image, where a number of pixels of the second image is not less than a number of pixels of the first image. Each pixel of the first grayscale image is matched with a pixel of the second grayscale image to create a pixel pair sequence. A similarity value indicating the similarities between the first image and the second image is calculated according to the grayscale value of each pixel in the pixel pair sequence S.